The invention relates generally to vacuum retention for a superconductive machine and, more particularly, to cryogenic cooling and vacuum retention for a high temperature superconductive (HTS) machine. As used here, the term “cryogenic” refers to a temperature less than about 150° Kelvin.
One exemplary superconductive machine is a superconductive rotor for electric generators and motors. Other superconductive machines include magnetic resonance imaging (MRI) systems, for medical applications, and magnetic levitation devices, for transportation applications. Typically, a superconductive coil assembly of a superconducting magnet for a superconductive device includes one or more superconductive coils wound from superconductive wire and which may be generally surrounded by a thermal shield. The assembly is typically contained within a vacuum enclosure.
Some superconductive magnets are conductively cooled by a cryocooler coldhead, such as that of a conventional Gifford-McMahon cryocooler, which is mounted to the magnet. Mounting of the cryocooler coldhead to the magnet, however, creates difficulties including the detrimental effects of stray magnetic fields on the coldhead motor, vibration transmission from the coldhead to the magnet, and temperature gradients along the thermal connections between the coldhead and the magnet. Such conductive cooling is not generally suitable for cooling rotating magnets, such as may constitute a superconductive rotor.
Other superconductive magnets are cooled by liquid helium in direct contact with the magnet, with the liquid helium boiling off as gaseous helium during magnet cooling and with the gaseous helium typically escaping from the magnet to the atmosphere. Locating the containment for the liquid helium inside the vacuum enclosure of the magnet increases the size of the superconductive magnet system, which is undesirable in many applications.
Superconducting rotors include a massive rotor core, which is typically at about room temperature, and a superconducting coil, which is in close proximity to the rotor core and which must be cooled below its operating temperature. The presence of impurities, such as gases, in the vicinity of the superconducting coil may cause ice build-up on the superconducting coil. The ice build-up over time may cause rub damage between moving superconducting coils and may further act as a thermal short between the rotor core and the superconducting coil(s).
Accordingly, superconducting machines, such as superconducting rotors, typically require vacuum insulation of the superconducting element(s) thereof, for example maintaining a vacuum for the superconducting coil(s). One known solution is to warm the superconducting coil, to room temperature, for example, to desorb the gases, which are adsorbed on the surface of the superconducting coil during operation thereof. However, this required shut-down and maintenance is undesirable for commercial power generation applications.
Accordingly, it would be desirable to provide innovations in a superconductive machine for operations over extended periods of time without regeneration. More particularly, it would be desirable to prevent or reduce the adsorption of gases on the superconducting element(s) of the machine, for example on the superconducting coil(s).